tropospheric ozone as a fungal elicitor.pdf

Tropospheric ozone as a fungal elicitor 125

J. Biosci. 34(1), March 2009

1. Introduction

Tropospheric ozone (O3) is recognized as the most phytotoxic

among the common air pollutants (Sandermann et al 1998),

and is responsible for a wide range of damages to plants. The

chemical characteristics at the basis of its behaviour are the

high oxidizing power, a diffusion coef cient similar to the

one of CO2 (and consequently a certain facility to penetrate

plant tissues), solubility in water 10 times higher than CO2

and tendency to react with water in a sub-basic environment

(Izuta 2006).

Its noxious activity towards plants can occur in both

direct (e.g. through liberation of hydroperoxides) and indirect

ways (e.g. through liberation of hydroximetylperoxides);

these aspects will be discussed in detail in the following

sections.

O3 is formed in the troposphere by the energy released

from electrical discharges, or can go down from the

stratosphere; nevertheless, it is mainly generated through

the photolytic cycle of O3, a series of chemical reactions

triggered by hydrocarbons and nitrogen oxides present in

exhaust gases from vehicles (Crutzen 1973; Chameides

and Lodge 1992). The cycle is started by the hydroxylic

radical, a highly reactive molecule formed when a radical

oxygen, generated spontaneously in the stratosphere by

splitting of O3, reaches the troposphere and there reacts with

H2O. At the same time, the hydroxylic radical can oxidize

anthropogenic pollutants to smaller chemical specimens that

are more easily eliminated. The synthesis or degradation of

O3 depends on the NO

2/NO ratio: the higher the ratio, the

higher the O3, and vice versa.

O3 concentrations in the troposphere regularly exceed

national and international limits in Europe and North

America (Hough and Derwent 1990; Flaty et al 1996),

ranging typically between 20 and 60 nl l-1 with peaks of up

to 250 nl l-1 (Stockwell et al 1997), and some models predict

a further increase of 0.31% per year over the next 50 years

(Liao et al 2006).

Under the stimulus of the environmental problems

connected with tropospheric O3, many researchers have

focused, during the past decades, on the study of its effects

on plants, and several properties that can be utilized for

convenient practical application have been brought to light

(Eckey-Kaltenbach et al 1994; Sudhakar et al 2006).

http://www.ias.ac.in/jbiosci J. Biosci. 34(1), March 2009, 125138, Indian Academy of Sciences 125

Review

Tropospheric ozone as a fungal elicitor

PAOLO ZUCCARINI

Department of Crop Biology, Section of Plant Physiology, University of Pisa, Pisa, Italy

(Fax, 0039 050 2216532; Email, [email protected])

Tropospheric ozone has been proven to trigger biochemical plant responses that are similar to the ones induced by

an attack of fungal pathogens, i.e. it resembles fungal elicitors. This suggests that ozone can represent a valid tool

for the study of stress responses and induction of resistance to pathogens. This review provides an overview of the

implications of such a phenomenon for basic and applied research. After an introduction about the environmental

implications of tropospheric ozone and plant responses to biotic stresses, the biochemistry of ozone stress is analysed,

pointing out its similarities with plant responses to pathogens and its possible applications.

[Zuccarini P 2009 Tropospheric ozone as a fungal elicitor; J. Biosci. 34 125138]

Keywords. Elicitor; ozone; pathogens; stress

Abbreviations used: ACC, 1-aminocyclopropyl-1-carboxylic acid; AOX, alternate oxidase; ATP, adenosine triphosphate; GRAS, generally

considered as safe; HR, hypersensitive response; NADH, nicotinamide adenine dinucleotide; NADPH, nicotinamide adenine dinucleotide

phosphate; NO, nitric oxide; NOS, NO synthase; PR proteins, proteins related to pathogenesis; PS, photosystem; ROS, reactive oxygen

species; SA, salicylic acid; SAR, systemically acquired resistance; SMV, Soybean mosaic virus; US, ultrasound; UV, ultraviolet

Paolo Zuccarini126


This review focuses on the capacity of tropospheric O3

to trigger the same plant reactions as fungal pathogens, and

on the possible implications of these for agronomy and plant

pathology. First of all, the reactions that plants mount as a

consequence of biotic stresses are discussed. Subsequently,

the damaging effects caused by O3 stress on the vegetable

organism are analysed. Finally, a comparison between the

two kinds of phenomena is performed, in order to highlight

the activity of O3

as a fungal elicitor, and its possible

practical applications.

2. Plant responses to biotic stresses

When the interaction between a pathogen and its host is

species-speci c, the host expresses genes of resistance with

biochemical af nity to the genes of avirulence brought by

the pathogen (the elicitors). When elicitors are recognized by

the proper receptors of the plant, they trigger the activation

of its genes of resistance, and defences are put into action

(Hahn 1996; Montesano et al 2003).

Elicitors present a wide range of structures, and can be

carried by the pathogen (exogenous elicitors) or produced by

the plant as a consequence of the plantpathogen interaction

(endogenous elicitors); in both cases, their role is to stimulate

the defence reaction of the plant (Ebel and Cosio 1994). The

most frequent biochemical defence responses are fast death

of those cells that are directly in contact with the pathogen

(known as hypersensitive response [HR]); synthesis and

accumulation of phytoalexins and secondary metabolites;

systemically acquired resistance (SAR); synthesis of

proteins related to the pathogenic event (PR proteins).

2.1 Hypersensitive response

HR is often the main mode of resistance in the case of fungal

or bacterial attack, and is characterized by the formation of

necrotic lesions in the zones involved and limitation of growth

and spread of the pathogen.

HR is mediated by reactive oxygen species (ROS) such

as superoxide, hydrogen peroxide and the hydroxyl radical

(Watanabe and Lam 2006). These activated species produced

by the plant undergoing an attack (Apel and Hirt 2004) can

directly kill the pathogens, strengthen the cellular walls by

deposition of structural compounds such as lignin, produce

speci c structural or elicitor proteins or destroy the host cell

(Baker and Orlandi 1995; Lamb and Dixon 1997).

The destruction of host cells is properly called a

hypersensitive response, and is caused by the peroxidation of

membrane lipids and loss of electrolytes. Cell ion imbalance

and the subsequent breakdown of cellular components result

in death of the affected cells and formation of local lesions.

In this way, the plant sacri es parts of its tissues, but

isolates the area of action of the pathogen and prevents it

from spreading through the whole organism (Heath 2000).

The reinforcement of cell walls surrounding the

infection is intended to create a physical barrier to inhibit

the spread of infection (Pontier et al 1998); this happens

through ROS-triggered deposition of callose and oxidized

derivates of precursors of lignin, as well as by production

of hydroxyproline-rich glycoproteins (Matthews 2007).

Speci c proteins produced in connection with HR can have

a structural or an elicitor role.

The role of ROS in pathogen killing and host cell

destruction has been widely demonstrated and accepted,

but evidence exists that they do not act alone (Dangl 1998).

Nitric oxide (NO) and NO synthase (NOS) play an important

role in plant defence reactions against pathogens, together

with ROS. Tobacco plants infected with a tobacco-speci c

virus showed an enhancement of NOS activity (Durner et

al 1998), and similar results were observed in soybean cells

and Arabidopsis thaliana in response to a bacterial pathogen

or a proper elicitor (Delledonne et al 1998), suggesting that

NO signi cantly contributes to the actions performed by

ROS in the early stages of plant defence responses.

HR is usually the rst defence of the plant against

pathogenic attack; when it is not suf cient to stop the

aggression, synthesis of phytoalexins comes to the rescue.

2.2 Synthesis of phytoalexins

Phytoalexins are a group of phenylpropanoids whose syn-

thesis is induced by various kinds of stress; phenylpropanes

come from deamination of the amino acids phenylalanine

and tyrosine produced in the biosynthetic pathway of

shikimic acid (Hammerschmidt 1999). About 350 different

phytoalexins are known today, all coming from the same

common metabolic pathway; plants belonging to different

botanical families synthesize speci c phytoalexins (Mert-

Trk 2002). Phytoalexins often do not come from synthesis

ex novo, but from the biotransformation of previously

existing molecules, e.g. by conjugation, compartmentation,

release of conjugated forms (Fuchs et al 1983; Pedras et

al 2000); this means that bigger stores of biotransformable

substances represent higher potential resistance for a plant.

Phytoalexins perform their antibiotic activity through

the destruction of pathogenic membranes, particularly the

plasmatic ones; this action is stronger for higher levels of

lipophilicity, hydroxylation and acidity of the molecule

(Cowan 1999; Ishida 2005).

2.3 Systemically acquired resistance

This phenomenon occurs when pathogen attack on a

certain part of the plant causes induction of resistance



in areas that have not been directly infected (Ryals et al

1996). The activation of systemically acquired resistance

(SAR) is constituted by a modulation or strengthening

of speci c mechanisms of resistance that cause the

plantpathogen interaction to be incompatible. However,

SAR cannot provide complete resistance from the attack

of pathogens, since for each plantelicitor combination a

particular induction of resistance, with its own spectrum, is

generated.

SAR is divided in two phases: start (the transitory

phase comprising all the events leading to resistance) and

maintenance (semi-stationary state of resistance coming

after the initial part).

Salicylic acid (SA) plays a central role in the signal

transmission for induction of SAR; in several species, a

correlation has been demonstrated between concentration

of SA and increased resistance to biotic stresses (Mur et al

1996; Mauch-Mani and Mtrauxs 1998; Mtrauxs 2001). SA

is produced in the infected tissues and transferred through

the phloem to non-infected ones, where it induces resistance,

acting as a signal molecule. For induction of resistance,

SA is not only synthesized ex novo at the moment of the

infection, but plants also have supplies of its conjugated

form with glucose, which can be released when necessary.

The accumulation of these conjugated forms represents a

constitutional defence for the plant.

Other molecules capable of conferring SAR are 2,6-

dichloroisonicotinic acid and its methyl ester (Vernooj et

al 1995); the S-methyl ester of benzo (1,2,3) thiadiazol-7-

carboxylic acid (Kunz et al 1997); jasmonate and its methyl

ester (Repka et al 2004).

2.4 Proteins related to pathogenesis

The contribution of proteins related to pathogenesis (PR

proteins) to disease resistance is highly variable, and depends

both on the plant and on the pathogen (Bowles 1990).

PR proteins accumulate in hostile environments as

vacuoles in the cell walls and intercellular spaces, since their

physicochemical proprieties allow them to resist low pH and

proteolytic scission (Datta and Muthukrishnan 1999). Their

basic role is to limit the access of pathogens and to induce

programmed cell death. The types and roles of PR proteins

are discussed further in section 4.

3. Biochemistry of ozone stress

When the plant is attacked by O3, it puts into action a

series of metabolic responses that can result in either

induction of resistance or damage. Damage can be either

acute or chronic. In this section, damage is analysed, with

special regard to the chemical specimens that activate

responses connected to it.

3.1 Acute damage

Acute damage can be de ned as damage subsequent to acute

exposure of an organism to a biotic or abiotic stressor. The

exposure is acute when it lasts for a short period of time,

during which the organism undergoes severely intense

stress. In the case of an abiotic stressor such as O3, acute

exposure is de ned as exposure to a high concentration of O3

for a short interval which is not repeated later; an example

of acute exposure could be 250 nl l-1 for 5 h (Pasqualini

et al 2007). Complexively, the three factors that interact

to distinguish between acute and chronic damage are the

severity and duration of the stress, and sensitivity of the

attacked organism.

Acute and chronic damage therefore have different

dynamics, and usually lead to different consequences.

While chronic exposure can provide the organism with

an increase in tolerance and resistance, in acute exposure

the modi cations caused to the metabolism of the host

are represented most of the time by the damage itself,

which in extreme cases can lead to death of the organism,

or to partial damage of the attacked tissues. This second

eventuality, in plants, often results in foliar necroses, and

can be a valid strategy to con ne the noxious agent and

prevent it from spreading to the rest of the organism. There

are also cases in which acute O3 exposure can provide the

plant with better resistance to further pathogenic attacks,

through induced metabolic changes that can persist for days,

weeks or months (Sandermann 2000). Puckette et al (2007)

suggest that acute O3 fumigation (300 nmol mol-1 for 6 h)

on Medicago truncatula can be a valid tool to improve the

plants resistance to a variety of abiotic stressors, in spite of

the damages (mainly at foliar level) caused by the treatment.

For this reason, varieties with high tolerance to acute O3

represent the ideal target for this kind of treatment. Soybean

plants exposed to acute O3 fumigation while being infected

with Soybean mosaic virus (SMV) showed acquisition of

non-speci c resistance against the virus which is concretized

by a signi cant slowing down of systemic infection and

disease development, by means of increased transcription

of fungal, bacterial and viral defence-related genes (Bilgin

et al 2008).

Acute damage is therefore the kind of damage that

generally, but not always, leads to cell death. Commonly

visible symptoms are foliar necroses, which can appear

within 1572 h from a single acute dose of O3 (Wohlgemuth

et al 2002; Neufeld et al 2006). These foliar necroses appear

similar to cases of programmed cell death subsequent to

pathogenic attacks, described by Kombrink and Somssich

(1995).

The necrotic lesions associated with acute damage

can either be caused by direct action of O3, leading to

oxidation of cellular components and uncontrolled cell

death (Pell et al 1997), or by programmed cell death

triggered by the ROS produced by primary reactions

(Greenberg et al 1994; Overmyer et al 2005), depending on

the O3 concentration. A high concentration of O

3 causes a

rapid attack on the cell walls and membranes, with loss of

semipermeability, plasmolysis and death. If the event is fast

and extensive, the quick and massive liberation of enzymes

from the tonoplast could lead to uncontrolled proteolysis and

uncontrolled cell death (Heath 1987b; Fiscus et al 2005).

Lower O3 concentrations can lead to a slow degradation of

the plasmatic membrane (2448 h). This happens through

ATP-ase inhibition (Dominy and Heath 1985), alteration of

Ca2+ transport (Castillo and Heath 1990) and oxygenation of

the membrane lipids (Ranieri et al 1996).

The key molecules in acute damage are ROS and

products of lipid peroxidation which, in cases of O3 stress,

seem to act both as messengers of stress signals (induction

of activity of detoxifying systems) (Puckette et al 2007) and

as being directly responsible for the development of necrotic

lesions (Mehdy et al 1996; Langebartels et al 2002). The

fact that the same molecule can play both these roles at the

same time cannot be ruled out, and the effects can depend

on its localization in the plant. For example, tobacco

plants modi ed for the overexpression of mitochondrial

alternate oxidase (AOX), characterized for this reason

by lower mitochondrial ROS concentration, surprisingly

showed higher sensitivity to O3 fumigation, and a possible

explanation is that ROS-scavenging systems were activated

by the altered defensive mitochondrial-to-nucleus signalling

pathway (Pasqualini et al 2007).

Products of lipid peroxidation are generated by the action

of ROS on the polyunsaturated fatty acids of the membrane

lipids (Takamura and Gardner 1996); their direct impact on

plant metabolism involves a series of reactions that take place

at the level of chloroplasts and their antioxidant systems

(Kraus et al 1995; Mano et al 2001) and at mitochondrial

level, where the glycine decarboxylase complex is a major

target (Taylor et al 2002).

In acute damage, O3 enters the plant through the

stomata, diffuses in the apoplast and, once there, is rapidly

decomposed to hydroxylic radical, superoxide, hydrogen

peroxide and other ROS (Heath and Taylor 1997). Formation

of the hydroxylic radical is stimulated by the presence

of Fe2+, amines, thiolic groups, caffeic acid (Grimes et al

1983; Byovet et al 1995); hydrogen peroxide is produced

by the reaction of O3 with unsaturated fatty acids (Pryor and

Church 1991).

These ROS can be detoxi ed by antioxidant substances

present in the apoplast and in plant mitochondria (Mller

2001); otherwise, they attack the proteins and lipids of

the plasmatic membrane through the process of lipid

peroxidation (Schraudner et al 1997). Lipid peroxide

radicals trigger various chain reactions, and the increased

level of conjugation of fatty acids reduces the elasticity

and uidity of the membrane, creating eccentricities

inside (Heath 1987a). Lipid peroxides can be detoxi ed

enzymatically by means of hydrolysis operated by

phospholipases (Chandra et al 1996; Schraudner et al 1997),

while glutathione-S-transferase and glutathione peroxidase

in the cytosol detoxify the secondary products of lipid

peroxidation (Willekens et al 1994; Conklin and Last 1995);

their activation is very fast after O3 treatment. Studies on the

biphasic production of ROS in O3-treated plants, moreover,

provide evidence for the contribution of plant endogenous

H2O

2 in the development of necrotic lesions connected to O

3

stress (Schraudner et al 1998; Castagna et al 2005).

Some authors also theorize that O3 can cause acute

damage through interaction with volatile molecules

produced by the plant, which are concentrated in the

apoplast, such as ethylene, isoprene and -pinene (Hewitt

et al 1990). Wellburn and Wellburn (1996) showed that

O3-sensitive plants had a higher ethylene production than

the average, while tolerant plants tended to accumulate

antioxidants such as polyamines, polyphenols, ascorbate

reductase and glutathione reductase. Other research, on the

other hand, provided evidence of a protective action of the

above-mentioned molecules against O3 stress (Loreto and

Velikova 2001). Cyanide, a secondary product associated

with ethylene formation, is considered in sensitive plants to

be an ulterior cause for necrosis (Grossmann 1996).

Acute O3 exposure negatively affects the photosynthetic

performance of the plant, mainly by inhibiting the

functionality of the photosystems (PS). Strong reduction in

photosynthetic activity, accompanied by a drop in stomatal

conductance, were observed in both ozone-sensitive and

ozone-tolerant tobacco cultivars after one single acute

fumigation (300 ppb during 3 h); these reductions were

reversible in the tolerant cv. and irreversible in the sensitive

one, demonstrating how damage caused to the PS can be

relatively easy to recover in tolerant plants (Pasqualini

et al 2002a). The effects of O3 and fungal pathogens

on photosynthesis are similar and, in both cases, affect

mainly PS-II, but some differences exist. Inhibition of

photosynthesis induced by O3 and by a necrotrophic fungal

pathogen, Pleiochaeta setosa, was compared in white lupin

leaves by chlorophyll imaging. In both cases, PS-II was the

main target of the perturbations, but the damage caused by

O3 became evident in a signi cantly shorter time than in

the case of the fungus; moreover, the spatial patterns of the

response on the surface of the leaves were totally different

for the two elicitors (Guidi et al 2007).

3.2 Chronic damage

This type of damage is caused by long-term exposure to low

O3 concentrations; a reduction in tness and competitiveness

Paolo Zuccarini128


are usually associated with it. The most common symptoms

are premature senescence, alteration in the metabolism of

sugars, inhibition of photosynthesis, loss of balance in the

redox status and production of ROS in the stroma (Pell et

al 1997).

Acceleration of leaf ageing is the most typical response

of plants to exposure to chronic doses of O3 (Pell and Dann

1991; Ljubei and Britvec 2006), and is associated with

an increased production of ethylene (Tingey et al 1976). No

cell death occurs, since the concentration of the oxidizing

agents is low enough to be tolerated by the plasmatic

membrane; nevertheless, free radicals tend to accumulate

over time, since the detoxifying enzymes of the plant

cannot completely clean them. This shows that ageing is

connected to an increase in oxidizing events and a reduction

in the capacity to counter them (Sohal and Weindruck

1996). There is a link between leaf ageing and trends in

the concentration of Rubisco in plant tissues. As a matter

of fact, treatment with O3 causes a reduction in the peaks

of Rubisco (Dann and Pell 1989; Pell et al 1992, 1994;

Kopper and Lindroth 2003) and accelerates the degradation

of the protein (Eckardt and Pell 1994); this suggests that

the drop in Rubisco levels, and its consequent decline in

photosynthesis, can be one of the ways through which O3

causes foliar senescence.

Ethylene is often associated with senescence too, but

its precise role is not yet clear, apart from the fact that

it is a promoter (Miller et al 1999). The most accepted

hypothesis considers ethylene to have a direct role both

in accelerating the process of senescence (Reid 1989) and

in acting as a signalling molecule (Guo and Ecker 2004;

Setyadjit et al 2004). The role of ethylene in the process of

senescence is closely related to that of polyamines (Pandey

et al 2000). The actions of these two classes of molecules

are sometimes complementary and sometimes antagonistic,

depending on the speci c physiological phase undergone by

the plant. Yang et al (2008) provided evidence that ethylene

plays a key role in the regulation of several developmental

processes connected to leaf senescence, such as petal

necrosis and corolla abscission on transgenic Nicotiana

sylvestris specimens. Woltering et al (2002) showed on

tomato plant cells how the action of ethylene in inducing

programmed cell death during leaf senescence can be

enhanced by the concomitant application of camptothecin,

an inducer of apoptosis, but the supply of ethylene alone did

not lead to signi cant alterations. Karaivazoglou et al (2004)

monitored the production rates of ethylene during ripening

and senescence of tobacco leaves, showing how an increase

in ethylene production coincided with the rst symptoms of

leaf senescence such as chlorophyll breakdown and decrease

of dry weight; the concentration peak was reached about one

week after the start of the process and was 56-fold higher

than basal ethylene production.

Alteration of sugar metabolism occurs in very sensitive

plants (such as deciduous trees), with accumulation of

starch in the guard cells and decrease of starch in the

mesophyll (Gnthardt-Goerg et al 1997). The accumulation

of starch and other hexoses inhibits the activity of several

enzymes involved in the Calvin cycle and photosynthetic

ef ciency is reduced (Krapp et al 1993). The reduced CO2

xation increases the pool of reducing equivalents, with the

consequent direct reduction of O2 by PS-I (Mehler reaction).

Therefore, the loss of balance in the redox status is the cause

of liberation of ROS into the stroma (Melhorn et al 1990;

Mittler et al 2004).

Chronic O3 exposure is commonly known to affect plant

photosynthesis (Heath 1994), and this involves a variety

of mechanisms that can act separately or in combination,

depending on the plant species and on the conditions of

exposure. A central role in the inhibition process is played

by the stomata (Incln et al 1998), the partial closure of

which represents the most immediate form of response, but

subsequent metabolic regulations follow, bringing about

a general limitation of physiological performance and a

decline in plant productivity.

Chloroplasts are the most important targets of chronic

O3 exposure: characteristic symptoms are represented

by alterations in their size and functionality, and in the

composition of the stroma. Both O3 fumigations and

natural tropospheric concentrations induced signi cant

size reductions in the chloroplasts of needles of Scots pine

and Norway spruce, and an increase in the electron density

of the stroma, especially on the upper side of the leaves

(Kivimenp et al 2005). Serious chloroplast injuries due to

oxidative stress were observed in sensitive clones of Betulla

papyrifera after O3 exposure, with subsequent dramatic loss

of functionality; no ultrastructural injuries were observed in

tolerant clones, as O3-elicited H

2O

2 production is restricted

to the apoplast (Oksanen et al 2004).

A typical response of plants to chronic exposure to O3 is

represented by a decrease in the quantum yield of electron

transport. This strategy allows the plant to reduce the

photosynthetic assimilation rate at analogous conditions of

irradiation, and is probably intended to reduce adenosine

triphosphate (ATP) and nicotinamide adenine dinucleotide

phosphate (NADPH) production in order to put the plant in

equilibrium with the decreased demand for the Calvin cycle

subsequent to O3 stress. Evidence of this fact is provided by

a trial in which chronic O3 fumigation on poplar (60 nl l1 for

5 h day1 over 15 days) resulted in a signi cant reduction in

the CO2 assimilation rate, due not only to strong stomatal

closure but also to limitation of the dark reactions of the

photosynthetic process, and the connected downregulation

of photosynthetic electron transport (Guidi et al 2001).

As can be seen, a problem for plants exposed to chronic

O3 concentrations is to reduce the photosynthetic rate, tune



Paolo Zuccarini130


it with the changed metabolic demands, and put into action

different alternative strategies for reducing equivalents.

Another possible mechanism of the photosynthetic

response can in fact involve more precocious steps, such

as the inhibition of PS-II, with no alterations in quenching

parameters (Guidi et al 2001). Ranieri et al (2001) showed

in poplar how chronic fumigation with O3 can induce

alterations in tylakoid functionality and composition; the

activity of both the photosystems (PS-II and PS-I) was

signi cantly reduced, and so was the concentration of all

the polypeptides analysed. This provides evidence of the

fact that, at a chronic level, O3 generally inhibits the activity

of the electron transport chain by lowering the PS protein

and pigment content, and all of these are strategies to

reduce the rate of photosynthetic activity to face the adverse

conditions.

Some similarities exist between the effects caused on

the photosynthetic process by chronic O3 exposure and

fungal pathogens. Carter and Knapp (2001) analysed a

large amount of published and unpublished data to show

that, among others stressors, fungal pathogens and O3 cause

alterations in the optical properties of leaves at almost the

same wavelengths.

The results shown provide evidence of an interaction

between the primary metabolism and the possible responses

to O3 stress, causing changes in the cell biochemistry,

structure of chloroplasts and composition of their proteins,

levels of reducing substances (such as nicotinamide adenine

dinucleotide [NADH] and other reduced equivalents) and

the redox balance of glutathione and ascorbate in the stroma.

These changes can affect plant productivity, shift certain

metabolic pathways (e.g. the shikimate way, Schmid and

Amrhein 1995) and alter the capacity of the plant to react to

future biotic and abiotic stresses.

O3 seems complexively to not cause direct damage

to the plant, as already theorized by Heath (1994), but to

activate several signal pathways in it. In this sense, O3 takes

shape of an abiotic elicitor, capable of stimulating plant

reactions that are similar to the ones derived from pathogen

attacks. These responses can either cause damage or act as

the basis for SAR; the discriminating factors are mainly the

intensity of the exposure and the individual sensitivity of the

subject.

4. Ozone as a fungal elicitor

In the previous section, plant responses to O3 stress

were analysed from the point of view of the kind of

damage induced. This section highlights all the cases

in which O3 elicits reactions comparable to the ones

generated by a fungal pathogen, potentially inducing the

plant to resist abiotic stresses. This mechanism is called

cross-induction.

4.1 Phytoalexins

Several experiments conducted during the past 30 years

demonstrate the capacity of O3 to stimulate the production

and accumulation of phytoalexins in different species.

O3 induces accumulation of iso avonoid phytoalexins

in soybean plants (Keen and Taylor 1975); of stilbenic

phytoalexins in pine (Sandermann 1996; Chiron et al 2000)

and grape (Schubert et al 1997) (the induction occurs at the

level of transcription, Zinser et al 1998); of catechins in

spruce and pine (Koricheva et al 1998); of the phytoalexins

of furanocoumarin in basil (De Moraes et al 2004). In

conifers, catechins and stilbenes can be accumulated and

stored for several months (Langebartels et al 1998), a

phenomenon called memory of the ozone stress.

The molecular mechanisms through which O3 induces

phytoalexin biosynthesis and the genes involved are still

an object of investigation. In leaves of Phaseolus vulgaris

L. exposed to realistic O3 doses, an increase was observed

in RNA accumulation of phenyalanine ammonia lyase,

naringenin chalcone synthase and chalcone isomerase

genes. The substances produced by these genes are involved

in the synthesis, among others, of iso avonoid phytoalexins

(Paolacci et al 2001). Grimmig et al (2004) provided

evidence of the fact that at least two different signalling

pathways for O3-induced gene expression are involved, one

depending on ethylene and the other an independent one.

The role of O3 in inducing phytoalexin biosynthesis

can also be exploited in the control of postharvest decay

of fresh fruit. In 1997, an expert panel declared O3 to be

generally considered as safe (GRAS) for applications

involving food contact (US FDA 1997). In table grapes, O3

was demonstrated to induce resveratrol and pterostilbene

phytoalexins, providing better resistance of the berries to

subsequent infections with Rhizopus stolonifer (Sarig et al

1996).

4.2 Cellular barriers

O3 exposure can provide plants with a reinforced cellular

structure, particularly by strengthening the cell walls. This

effect is generally observed at foliar level and is connected

in most cases with an enhancement of lignin production and

deposition, and with the switching of the related metabolic

pathways towards the biosynthesis of particularly resistant

and exible species of lignin.

O3 induces, usually at transcriptional level, the activity

of cinnamyl alcohol dehydrogenase (Zinser et al 1998;

Soldatini et al 2005) leading to the production of a lignin

with juvenile characteristics that is closely correlated

to the extensins (Lange et al 1995). This modi cation

results in more resistant cells, with more ligni ed and

elastic walls. In parsley (Eckey-Kaltenbach et al 1994)



and tobacco (Sandermann 1996), O3 induces the synthesis

of callose. In soybean plants, O3 treatment induces

modi cations in the metabolism of phenylpropane and in

the phenolic composition of leaves, leading to a higher

content of hydrocinnamic acid, lignin and suberin. This

was, again, mainly due to the increase in cinnamyl alcohol

dehydrogenase activity, but O3 also elicited reactions

typically associated with wound responses and browning

(Booker and Miller 1998).

O3 fumigation of poplar trees induced signi cant incre-

ase in the foliar activities of shikimate dehydrogenase,

phenylalanine ammonia lyase and cinnamyl alcohol

dehydrogenase, enzymes involved in various steps of the

metabolic pathway for the biosynthesis of lignins. A higher

proportion of Klason lignin was observed in extract-free

leaves of treated plants, and the lignins synthesized in

response to O3 showed a different structure with regard to

pre-existing lignins, with more juvenile characteristics such

as enrichment of carboncarbon interunit bonds and in p-

hydroxyphenylpropane units (Cabane et al 2004).

The data shown here suggest that O3 fumigation can

provide the plant with higher tolerance to O3 itself and

to fungal pathogens by inducing, among other effects, a

substantial reinforcement of the cell wall by synthesis of

higher amounts of lignins that can provide better mechanical

performance than constitutive ones.

4.3 PR proteins

Fungal and viral infections are responsible for an increase

in PR proteins among the soluble protein fraction of leaves

of most plants (Bol et al 1990; Bowles 1990); O3 is capable

of triggering the same response. The common factor in

the stimulation, in the case of both O3 stress and attack by

pathogens, is the liberation of ethylene (Sandermann 1996;

Van Loon et al 2006), and the molecules whose production

is stimulated more are glucanase, chitinase and glutathione-

S-transferase-1.

Both the stimulation and increase of pre-existing

production of PR proteins have been demonstrated in

tobacco (Ernst et al 1992; Ernst et al 1996; Yalpani et al

1994; Van Buuren et al 2002), parsley (Eckey-Kaltenbach

et al 1997), Arabidopsis (Sharma and Davis 1994; Conklin

and Last 1995; Sharma et al 1996; Lim et al 2003) and

spruce (Krenlampi et al 1994) treated with O3. RNA-blot

analysis performed on O3-tolerant and O

3-sensitive clones

of hybrid poplar showed that O3-induced mRNA levels of

O-methyltransferase, a PR protein, were signi cantly higher

in the O3-tolerant clones (Riehl Koch et al 1998). Long-term

induction of genes encoding stress-related proteins PR-10

and PAL was related, in birch, to macroscopic symptoms of

injury (necrotic ecks) and enhanced yellowing of leaves,

and leaf injuries were connected with short-term stomatal

closure response in a highly complex manner (Pkknen

et al 1998). O3-induced production of PR proteins can be

enhanced by the concomitant action of other abiotic factors:

in potato plants susceptible to Phytophthora infestans, an

increase in the constitutive activities of the PR proteins

-1,3-glucanase and osmotin is mediated by the combined

action of high O3 and CO

2 concentrations, resulting in an

improved resistance to the pathogen (Plessl et al 2007).

4.4 Signal substances

These substances have a role in transmitting information

from the areas attacked by O3 to the rest of the plant to

activate the defences of the organism, such as PR proteins.

The message can be carried by a hypothetical O3 receptor, or

redox-sensor; by the oxidative burst at apoplastic and, later,

at symplastic level (Ernst et al 1992; Sharma and Davis

1994) or by other messengers.

The two most important signal molecules are ethylene and

SA; they can either be induced directly by O3 or be secondary

messengers. It was demonstrated that these two molecules

can act in concert to in uence cell death in O3-sensitive

genotypes and that, at the same time, O3-induced ethylene

production is dependent on SA (Rao et al 2002), and SA

production is regulated by ethylene (Ogawa et al 2005)

The production and circulation of ethylene as a

consequence of O3 stress has been studied in potato (Pell et

al 1997; Schlagnhaufer et al 1998; Sinn et al 2004), tomato

(Tuomainen et al 1998; Moeder et al 2002), Arabidopsis

(Overmeyer et al 2000) and birch (Vahala et al 2003),

and is testi ed by the activation of 1-aminocyclopropyl-

1-carboxylic acid (ACC)-synthase and ACC-oxidase (Yin

et al 1994; Glick et al 1995; Sandermann 1996; Moeder

et al 2002). The induction of ACC-oxidase transcription is

the fastest response to O3 in plants, occurring in less than

30 min from stimulation (Pell et al 1997; Tuomainen et al

1997). In tomato, exposure to O3 concentrations of 85 nl l-1

for 5 h caused visible foliar damage by 24 h, and the activity

of ACC-synthase started to increase after 2 h (Tuomainen et

al 1997); however, in tomato, changes in mRNA levels of

speci c ACC-synthase, ACC-oxidase, and ethylene receptor

genes occurred within 15 h of treatment (Moeder et al

2002). The effects produced by stress-induced ethylene are

typical of both HR and SAR reactions, such as synthesis of

PR proteins (Ernst et al 1992; Ernst et al 1996), synthesis

of stilbene synthase (Schubert et al 1997), accelerated

senescence (Pell et al 1997), inactivation of Rubisco (Glick

et al 1995) and modulation of programmed cell death (Lamb

and Dixon 1997; Greenberg 1997). Some studies also

suggest that ethylene may react non-enzymatically with O3

to give a superoxide radical, thereby directly determining

the responses of plants to O3 (Elstner et al 1985; Mehlhorn

and Wellburn 1987).

Paolo Zuccarini132


SA is considered a signal molecule capable of inducing

both HR and SAR responses (Lamb and Dixon 1997;

Durner et al 1997; Takashi et al 2006). Studies demonstrate

the induction of SA and of its -D-glycosidic conjugate

in tobacco (Yalpani et al 1994) jointly with an increase

in resistance to Tobamovirus, and on Arabidopsis,

with a concomitant induction of resistance to Pseudo-

monas syringae (Sharma et al 1996). SA also

has a role of messenger similar to that of ethylene,

mediating SAR responses such as induction of PR proteins

and lipoxygenases involved in the synthesis of jasmonic

acid, which prevents the visible symptoms of O3 stress

(Ernst et al 1992; Thalmair et al 1996; Eckey-Kaltenbach et

al 1997; Sharma et al 1996). In some genetically modi ed

organisms containing a bacterial salicylate hydroxylase,

SAR responses are signi cantly affected (Sharma et al

1996; rvar et al 1997). It is commonly accepted at present

that high SA content could trigger the production of ROS

with subsequent SA-mediated cell death (Pasqualini et al

2002b).

4.5 Antioxidative systems

An important effect of O3 as an elicitor is to stimulate

the synthesis and accumulation of several antioxidative

enzymes located in the apoplast and plasmatic membrane,

such as catalases, glutathione peroxidases, glutathione-

S-transferases (Sandermann 1996; Noormets et al 2000),

superoxide dismutase and ascorbate peroxidase; the latter

two usually have light and delayed effects (Willekens

et al 1994). The kind and severity of the antioxidative

response depends on the plant species, on the onthogenic

phase (Sandermann 1996; Heath and Taylor 1997) and on

the compartments involved (cytosol, chloroplast, apoplast)

(Sandermann 1996; Schraudner et al 1997; Van Hove et

al 2001), since each of them hosts different antioxidative

systems.

Ethylene has been shown to have an important role in

inducing HR in hypersensitive tobacco (Greenberg 1997);

when sensitive Arabidopsis was deprived of ascorbic acid

the noxious effect of ROS was detected (Conklin et al 1996).

Ascorbate has been studied for its detoxifying properties in

spinach (Luwe et al 1993), which plays a role both as a direct

antioxidant and reducer of -tocopherol that is activated in

this way, and in soybean (Robinson and Britz 2001), in

which it was shown to play a more important role than

dehydroascorbate in enhancing plant tolerance to elevated

levels of O3. Polyamines, both in their free and conjugated

forms, have been demonstrated to reduce the gravity of

lesions due to O3 in tobacco (Kangasjrvi et al 1994) by

inhibiting lipid peroxidation and preventing premature

senescence, and regulating adaptation of the photosynthetic

apparatus.

4.6 Other abiotic elicitors

Tropospheric O3 has been demonstrated to play an important

role as a fungal elicitor, but it is not the only chemical with this

action. Evidence has been collected over many years of the

possibility that other abiotic factors could trigger mechanisms

of plant reaction similar to the ones provoked by bacterial or

fungal pathogens. This is due to the fact that O3 and other

abiotic stressors can, in speci c cases, trigger analogous

metabolic mechanisms of response in the attacked plant, most

of which are mediated by the production of ROS, providing the

opportunity for interesting crossed applications. For example,

wounding prior to high exposure to O3 of tobacco reduced the

severity of injury caused by O3, because of overexpression

of the antioxidant enzyme ascorbate peroxidase due to the

mechanical stress (rvar et al 1997).

With regard to agents other than O3, a variety of biotic

and abiotic elicitors for the production of phytoalexins have

been identi ed (Darvill and Albersheim 1984). Davis et al

(1986) demonstrated on cotyledons of soybean plants that

the accumulation of phytoalexins, a typical plant response

to microbic aggressions, is favoured by the combined and

synergistic action of the elicitor-active hexa--glucosyl

glucitol, and various biotic and abiotic elicitors. Treatment of

cotyledons of Vicia faba with both ultraviolet (UV) radiation

and freezingthawing caused a remarkable increase in the

production of phytoalexins, particularly wyerone, giving

results comparable with those caused by a typical biotic

agent such as Botrytis cynerea (Soylu et al 2002).

Low-energy ultrasound (US) was demonstrated to

induce plant defence responses and increase the production

of several secondary metabolites in Panax ginseng cells

in suspension culture, effecting an elicitor-like effect.

In particular, increased cross-membrane ion uxes and

production of ROS were observed, as well as synthesis of

saponins (Wu and Lin 2002).

There is also evidence of overlap of the effects of O3

and other stressors, both biotic and abiotic, on plants. O3

treatment on parsley cell cultures resulted in simultaneous

induction of the pathways of phenylpropanoid metabolism,

usually associated with the action of fungal elicitors and

UV irradiation, respectively (Eckey-Kaltenbach et al 1994),

demonstrating how this gas can elicit a wide range of defence

responses in plants. Yalpani et al (1994) showed how both

O3 and UV light stimulated the production and accumulation

of SA and PR proteins in tobacco, increasing the resistance

against Tobacco mosaic virus.

O3 is therefore not the only agent capable of inducing

plant defence responses similar to those due to fungal attack.

It shares this activity with numerous biotic and abiotic

factors, but stands out for its ef cacy (Sandermann 1996,

2004) and the wide applicability of this property. This is

the reason why O3 fumigation is used successfully today



in a variety of agronomical applications, such as conferring

resistance against fungal pathogens; for example, against

Bipolaris sorokiniana in barley and fescue, against Phoma

lingam in rape (Pazek et al 2001) or against Botrytis cinerea

in strawberry plants (Nadas et al 2006).

5. Conclusions

The data presented here demonstrate how O3 shows the

typical characteristics of a fungal elicitor, which can be

utilized both with the objective of inducing resistance to

the attack of pathogens in plants and for the study of plant

defence reactions to the above-mentioned attacks. This

idea is feasible by virtue of the fact that O3 application is

economically convenient and technically easy to perform.

O3 can be used alone or in association with other

preparations such as active pathogens, fungal elicitors and

signal substances. In particular, O3 and ethylene are the only

elicitors that can be easily removed after each experiment.

O3 is the best among various substances for performing

the treatment, since it is the easiest to produce, apply and

remove. Several examples exist in the literature of the use of

O3 to induce resistance to fungal pathogens.

In conclusion, O3 is an important instrument for the study

of plant responses to biotic and abiotic stress, and a valid

alternative to more expensive and complicated treatments

for the induction of resistance to several pathogens, with no

particular environmental impact. However, this subject has

not been studied deeply enough yet, since each plant can

show a different set of responses to different applications

of O3; moreover, the constant increase in tropospheric O

3

in several parts of the world is causing a huge change on a

global scale. The two-way activity of O3, which is capable of

both predisposing plants to the attack of viruses, pathogens

and insects, and inducing resistance to these same factors,

depending on factors such as the kind of plant and the nature

of the exposure, makes us realize how the medium- and long-

term effects of this phenomenon are not easily predictable.

Further, capillary experimentation will be required.

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MS received 27 November 2007; accepted 3 November 2008

ePublication: 6 January 2009

Corresponding editor: VIDYANAND NANJUNDIAH

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